Restrict Inbound and outbound mail in MS Exchange

To restrict inbound Internet e-mail for specific users or for distribution groups.

Microsoft Exchange Server 2003 has a new feature that permits mailbox users or distribution groups to only receive e-mail messages from authenticated users. This feature permits you to restrict inbound Internet e-mail for specific users or for distribution groups. The feature is enabled when you click to select the From authenticated users only check box in the Message restrictions settings for an individual user or a distribution group.

Click Start, point to Programs, point to Administrative Tools, and then click Active Directory Users and Computers.

Right-click the user account, and then click Properties.

Click the Exchange General tab.

Click Delivery Restrictions.

Under Message restrictions, click to select the From authenticated users only check box.

For more information, please refer to the following article.
827616 How to restrict the users who can send inbound Internet e-mail to another user or to a distribution group in Exchange 2003
http://support.microsoft.com/?id=827616

No External Outbound E-Mail

To restrict Internet outbound e-mails

Simply create a group that will contain users that you want to be able to send internet email.

Add the users that you want to have access to internet email to the group created above.

Then, on your SMTP Connector, go to the delivery restrictions tab and set by default to reject all messages, but accept only the Group that was created above. Set the address space with an * that has a delivery restriction to reject messages from the local group” (as long as the group isn’t very large).

NOTE: By default, Delivery restrictions are not functional until set in the registry. Please see this KB article for more information.

Q277872 XCON: Connector Delivery Restrictions Do Not Work Correctly
http://support.microsoft.com/default.aspx?scid=kb;enus;Q277872&SD=MSKB&
For more information please refer to the part one “Restricting Users from Sending Internet Based Email” of Mark’s article below.
http://www.msexchange.org/tutorials/MF009.html
And you may refer to this article as well.
http://www.msexchange.org/tutorials/MF025.html